Automatic fidelity control circuits



April 5, 1938. R. A. BRADEN AUTOMATIC FIDELITY CONTROL-CIRCUITS Filed May 25, 1935 4- Sheets-Sheet 1 llllllll` April 5, 1938. R. A. BRADEN 2,113,395

AUTOMATIC FIDELITY CONTROL CIRCUITS Filed May 25, 1955 4 Sheets-Sheet 5 xNvENToR RENE A. ADEN Bf) a TToRNx-:Y

.SOURCE April 5, 1938.

70 MMI f 50x/H65 Eg. Al-

/A/PUT HEAT/Vf' OUTPUT R. A. BRADEN AUTOMATIC FIDELITYl CONTROL CIRCUITS Filed May 25, 1935 4 Sheets-Sheet 4 INVENTOR RENE BRADEN BY www ATTORN EY Patented Apr. 5, 1938 UNITED STATES aligns PATENT @ifiwiii Rene A. Braden, Collingswood, N. J., ,assigner te Radio Corporation of America, a corporation ci Delaware Application May 25, 1935, Serial No. 23,470

16 Claims.

My present invention relates to fidelity control arrangements for signalling systems, and more particularly to automatic fidelity control systems for radio receivers.

Radio broadcast receivers of present commercial types are, in general, the result of compromises in design between two mutually exclusive characteristics, e. g., sufficient selectivity to differentiate between incoming signals under maX- imum and minimum sensitivity conditions, and suicient fidelity to provide natural reproduction of the higher audio frequencies. While a fair degree of delity had been attained in the prior art through the use of inter-tube coupling circuits having band pass characteristics, it was considered difficult to design radio receivers, especially those provided with automatic volume control, that would exhibit a high degree of fidelity as well as reasonable selectivity, when receiving o strong signals, and still be sufficiently selective to receive weak signals without an unpleasant amount of interference and background noise.

In my copending application Serial No. 10,981, iiled March 14, 1935 Patent No. 2,053,762, of September 8, 1936, there are disclosed automatic fidelity control systems which involve the automatic regulation of the gain of sharp and broad amplifiers in such a manner that the gain of the sharp amplier decreases at a more rapid rate than the broad amplier when strong signals are received.

One of the main objects of my present invention is to provide improved automatic fidelity control circuits utilizing electron discharge tube amplifiers of special design, the amplifiers being operatively associated with signal transmission paths of sharp and broad selectivity characteristics, and the geometry of the amplier tubes being such that variations in received signal amplitude may be utilized to vary the sensitivityidelity characteristics of the signal transmission paths by varying the electronic flow through different portions of the amplifier tubes.

Another important object of the present invention is to utilize electron discharge tubes of the exponential, or variable mu, type as ampliers. the amplifier tubes being constructed to feed signal channels having different selectivity characteristics, and the signal transmission through the channels being regulated by automatic variation of the flow of parallel electron streams within each of the ampliiiers.

Another object of the invention is to provide various tube constructions which are readily adapted for use in connection with radio receiv- (Cl. Z50-Z0) tems which are not only reliable and efficient in operation, but economically constructed and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, theinvention itself, however, as to both its organization` and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indcated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. l diagrammatically shows a radio receiving circuit including an intermediate frequency amplifier system embodying various tube constructions according to the present invention,

Fig. 2 graphically represents the characteristics of the coupling network between tubes 9 and l@y of Fig. l,

Fig. 3 is a schematic representation of a tube construction which may be utilized for tube 9 of Fig. 1,

Fig. 4 is a schematic representation of a tube construction which may be utilized for tube I0 n Fig. 1,

Fig. 5 is a modification of a tube construction of the type shown in Fig. 4,

Fig. 6 is a schematic representation of a tube construction which may be used for tube l'l of Fig-1,

Fig. 7 is a schematic illustration of a modified tube construction which may be used for any of tubes 9, It and il of Fig, 1,

Fig. 8 is a circuit diagram of a signal transmission network employing variable mu tubes of modiiied construction,

Fig. 9 shows a modied form of coupling network which may be utilized with the present invention, y

Fig. l0 shows a modified arrangement for securing bias control in accordance with the present invention,

Fig. 1l illustrates a modification of the coupling network shown in Fig. 9,

Fig. 12 shows an audio frequency transmission network embodying the present invention,

Fig. 13 shows the audio transmission characteristics of the system of Fig. 12.

Referring now to the accompanying drawings, wherein like reference characteristics in the different figures correspond to similar circuit elements, there is shown in Fig. 1 a circuit diagram of a superheterodyne receiver which embodies a plurality of signal transmission networks, any of which may be utilized in practicing the present invention. The superheterodyne: receiver is of a conventional type in its essential elements, and may comprise a customary signal source l, such as a signal collector of the grounded antenna type. The source l may, also, include one, or more, stages of tunable radio frequency amplification, and the output of the radio frequency amplifier is impressed upon the usual frequency changer network. This network may comprise a first detector tube which is independent of the local oscillator tube; alternatively, the local oscillator and rst detector may be of the combined local oscillator-first detector type and utilize, if desired, a pentagrid converter tube of the ZAT type. It is to be clearly understood that the nature of the networks preceding the input of the intermediate frequency amplifier may be suited to the purpose of the set designer. The essential thing in the design of the receiver shown in Fig. 1 is to impress upon the input transformer M1, which has its primary and secondary windings each tuned to the operating intermediate frequency, intermediate frequency energy of a substantially constant frequency over the entire tuning range of the receiver. This frequency may be of 'any value desired, and may have a value, for example, of 450 kilocycles, as is now common practice.

The intermediate frequency amplifier includes a plurality of networks which will be described in detail at a later point. The output of the last intermediate frequency amplifier tube is transmitted to the second detector network through a transformer M2. The latter has its primary and secondary circuits tuned to the operating intermediate frequency. The second detector network utilizes a diode rectifier, and the electrodes of the diode are the cathode of tube 3 and the diode anode 4. The tube 3 is a tube of the multiple function type, and may be a 55 or 85 type tube. By way of illustration, and in order to simplify the drawings, the tube 3 is shown as including a single diode section and a triode section. Those skilled in the art will readily appreciate the fact that the triode section may be replaced by a pentode section, and that more than one diode section can be utilized. Between the diode anode 4 and the cathode of tube 3 there is connected in series the tuned input circuit 5 and the diode load resistor R. The resistor R is shunted by an intermediate frequency by-pass condenser 6.

The triode section of tube 3 functions as an audio frequency amplifier, and the signal grid thereof is connected to a desired point on resistor R through a series path which includes the condenser 'I and coil 8. The connection to the load resistor may be made adjustable so that the adjustable connection can function as a manual volume control device. An intermediate frequency by-pass condenser is connected between the signal grid of tube 3 and the positive side of resistor R, and the function of condenser 'I and coil 3 is to suppress the intermediate frequency component of rectified signal current. The audio frequency currents flowing in the plate circuit of tube 3 are transmitted to the following audio frequency network through the coupling transformer M3, and it will be understood that the audio network may comprise one, or more, additional stages of audio amplification, a reproducer being employed for utilizing the amplified audio energy.

In order to show the manner in which the various direct current energizing voltages are supplied to the tubes of the receiving system shown in Fig. 1, there is shown in Fig. 1 the D. C. supply source for the various electrodes of the electron discharge tubes of the system. It is not believed necessary to explain the manner of connecting the various electrodes of the tubes in the receiving system to the voltage supply bleeder P. rl'hose skilled in the art will readily recognize the manner of making these various connections from the designations noted on Fig. 1.

Considering now the specific constructions of the intermediate frequency amplifier system, it will be observed that it comprises four cascaded stages. The rst of these stages includes the electron discharge tube 9 which comprises in addition to the usual cathode, signal input grid, and screen grid, a divided plate. The representation of tube 9 in Fig. 1 is functional in nature; the specific construction of a tube of this type will be shown at a later point in the specification. It will be sufficient for the present to point out that the tube is one whose signal grid is constructed in such a manner as to impart a variable mu characteristic to the tube. In place of utilizing a signal plate in the normal manner the plate is divided into two parts, and the screen grid electrode not only shields the two anodes from the remaining electrodes, but also is constructed to shield the two anodes from each other. 'Ihe I. F. amplifier tube I0 following tube 9 differs in construction from the latter in that the signal input grid is also divided. The tube l0, however, is also of the variable mu type, and a designation has been shown in Fig. 1 adjacent both representations of the tube to designate that these tubes are of this specific type.

The intermediate frequency energy is impressed between the input electrodes of tube 9; the output energy of the tube is divided between two signal transmission paths. One of these paths includes the coupling transformer M4 which has its primary and secondary circuits each tuned to the intermediate frequency. The tuned primary circuit of transformer M4 is disposed in the plate current connection to the plate Il of tube 9, while the tuned secondary circuit of the transformer is connected in circuit with the grid I2 of tube I0. 'I'he co-efficient of y coupling between the primary and secondary circuits of transformer M4 is given a value such that the resonance curve characteristic of the network including the tuned circuits of transformer M4 will be broad and have a substantially iiat top. In other words, the coupling Value of the tuned primary and secondary circuits of transformer M4 is such that a broad band of signal frequencies will be transmitted through that network.

'I'he second signal transmission path between tubes 9 and I0 comprises transformer M5 which is provided with resonant primary and secondary circuits each tuned to the intermediate frequency. The tuned primary circuit of transformer M5 is connected in the plate current circuit to the plate I3 of tube 9, and the tuned sec- -ondary circuit of this transformer is connected in circuit with grid I4 of tube I0. The coupling between the primary and secondary circuits of transformer M5 is relatively loose, and is given a value such that a narrow band of signal frequencies will be transmitted through this network. In order to emphasize the difference in coupling of the circuits of networks M4 and M5 the spacing between the windings of these networks has been shown as different, and that between the windings of transformer M5 has been shown as further apart to denote that the coupling in this case is relatively loose. A resistor I5 is connected across the tuned primary circuit of transformer M4 in order to improve the wide band transmission characteristic of coupling network M4.

The amplier tube Ill is followed by a pair of screen grid tubes I6 and I6 arranged in parallel, the signal input circuit of tube I5 being coupled to the plate II of tube I through a coupling network Ms whose design is similar to that of coupling network M4. It will be noted that resistor I' is connected across the tuned primary circuit of coupling network M6 for the same purpose as in the case of resistor I5. The signal input grid of amplifier tube IB' is coupled to the plate I3 of tube I0 through coupling network M1 whose design is similar to that of coupling network M5. The nal intermediate frequency ampliiier tube I'I is a screen grid tube having a pair of divided grids, one of the grids I8 being connected to the plate circuit of amplifier le through a coupling network M8. The design of coupling network M8 is substantially similar to that of networks M4 and Ms. The signal grid I5 of tube II is coupled to the plate circuit of amplifier I5' through a coupling network M9 whose design is similar to that of networks M5 and MF1.

It will, therefore, be observed that co-upling networks M4, M5 and M8 each include circuits which are relatively closely coupled, whereas the coupling network M5, M7 and M9 include circuits which are relatively loosely coupled. In this way there is provided between the input circuit of the intermediate frequency amplier system and the output circuit thereof a pair of parallel signal transmission paths, one of the paths having a relatively broad selectivity characteristic, while the other path has a relatively sharp selectivity characteristic.

The signal transmission efficiency through these parallel paths may be differentially regulated by means of the automatic gain control connections provided between the signal input grid circuits of the various tubes of the intermediate frequency amplier system and a source of direct current potential which is responsive in amplitude to received signal amplitude variations. These gain control connections are denoted in heavy lines in Fig. l, and are generally designated by the symbol AGC. The connections are provided between each of the signal input grids of tubes 9, IU, I6, I6' and I'I and the negative side of diode rectifier load resistor R. It will be appreciated that as the received signal amplitude increases, the diode anode side of resistor R will increase in negative potential. Therefore, the negative bias on the signal grids of the various tubes of the I. F. amplifier system will increase. This increase in negative bias on each signal grid will affect the electron streams flowing through the signal grids in diierent manner. It is pointed out that the automatic gain control path includes proper resistor-condenser iilter networks for suppressing pulsating current components, and since such devices are well known to those skilled in the art, it is considered suicient to diagrammatically represent these lter networks in the circuit diagram of Fig. 1.

In order to clearly explain the functioning of the present invention, attention is directed to that portion of the I. F. amplifier system which includes tubes 9 and I and their coupling networks. In Fig. 2 there are graphically represented the resonance curve characteristics which are obtained by means of the present invention. These characteristics show that it is desired to have the broad band transmission characteristic between tubes 9 and I Il when receiving strong signals, while a narrow selectivity characteristic is secured when receiving weak signals. The nature of an intermediate characteristic is also depicted in Fig. 2 in order to show the effect of the resonance curve characteristics of coupling networks M4 and M5 when receiving signals of medium strength.

As stated heretofore tube 9 is a tube of the variable mu type, and one of the characteristics of a variable mu tube is that when the negative bias on the signal input grid is made sufficiently high, electron now is conined to one end of the electrode system, and the amplification is correspondingly reduced. This phenomenon occurs by reason of the fact that the windings of the signal input grid in such a tube are closely spaced at one end and relatively widely spaced at the other end. The closely spaced end exerts the greatest amount of control upon the electron stream, so that a relatively small negative bias is suicient to prevent electron iiow through this portion of the grid, while a much greater negative bias would be required to prevent flow of electrons through the widely spaced portion of the grid to the adjacent portion of the plate. The tube 9 has two plates so placed that one collects the current which ows through the closely spaced portion of the grid, while the other collects the current which ilows through the widely spaced part. The amplification of a signal in each plate circuit is controlled by the adjacent part of the grid, and varies with grid bias in the same way as the plate current.

The plate Ii of tube 9 is to be understood as being positioned in alignment with the widely spaced portions of the signal input grid, and this plate is coupled through network M4 to the input grid I2 of tube ill. It will, therefore, be seen that when the negative bias on the signal input grid of tube 9 is high, the coupling network M4l determines the selectivity of the portion of the I. F. amplifier system between tubes 9 and Ill, the coupling network M5 having substantially no effect since the electron flow to plate I3 is entirely cut oif. This is the state of affairs for securing the broad band characteristic shown in Fig. 2.

On the other hand when the bias on the signal input grid of tube 9 has its minimum negative value, as when the receiver collects weak signals, both plates II and I3 of tube S receive plate current and the signal is transmitted to grids I2 and Id of tube I0 through both coupling networks M4 and M5. In this case the component of the signal which is passed by way of plate I3 and grid Id has the selectivity characteristic of coupling network M5. However, the transmission through coupling network M4 is still operative, and, therefore, the total effective signal impressed upon the input grids of tube It is the sum of the signal components through thebroad and narrow band networks. The net effect, however, will be considerably sharper in selectivity, but reduced in fidelity, as compared with the first condition in which the loosely coupled network M5 does not function.

The curves shown in Fig. 2 illustrate in a qualitative manner the characteristics under the two extreme conditions of signal intensity, and under the intermediate condition, of signal transmission between tubes 9 and Il) of the intermediate frequency amplifier system. Since the broad band condition would ordinarily be required only when the gain is very low, it is possible to make the low gain section of tube 9 relatively small (that is to say the plate II would have a smaller area than plate I3), and this would make the circuit I3--M5--I4 predominate under high gain conditions to such an extent that the broadening effect of circuit II-M4-I2 on the net selectivity would be extremely small.

It is to be clearly understood that the geometry of the variable tube 9 may assume many diiferent forms. It has been explained that the variable mu eifect is imparted to tube 9 by using a signal input grid which has more widely spaced windings adjacent plate II. Those skilled in the art are well aware at the present time of other constructions which will secure the desired variable mu characteristic. By way of illustration there is shown in Fig. 3 a schematic representation of an electron discharge tube construction which may be employed for the functions of tube 9. It will be noted from this schematic showing that the electrode structure includes, in addition to the divided plates II and I3, a conical signal control grid G.

The screen grid S is provided with an extensive shielding flat ring S1 which functions to shield the plates II and I3 from each other. The symbol S denotes the support structure for the screen grid S, and the tube envelope is shown in dotted outline about the electrodes. The variable mu characteristic is secured in this case because one end of the grid is close to the cathode while the other end is far from the cathode, the screen diameter being uniform. It is not believed necessary to explain the mode of operation of this form of tube, since those skilled in the art are fully aware of the fact that the conical configuration of signal control grid G will impart the desired variable mu characteristic to the tube. It is also possible to employ a signal control grid of uniform diameter, and pitch, of winding from end to end. Then, the screen grid is made conical in formation to secure the Variable mu characteristic for tube 9. The plates i I and i3 may be arranged in a conical configuration to secure the same characteristic, in case the tube is a triode.

The amplifier tube I0 in Fig. l, differs in construction from tube 9 in that it additionally possesses divided grids I2 and I4. Fig. 4 shows a schematic representation of such a variable mu tube construction. 'I'he essential difference between the construction shown in Fig. 4 and that shown in Fig. 3 resides in the fact that the conical signal control grid is divided into two portions, and these portions correspond to the grids $2 and i4 of the tube I0 in Fig. 1. It will be observed that the screen grid is provided with the ring extension S2 between plates II and I4 for electrostatic shielding of these plate sections. The two grids could be shielded from each other by a ring tied to cathode.

Instead of using for the tube I signal control grids I2 and I4 arranged in the tapered fashion with separate leads to each section, a variable pitch grid winding can be used in place of the variable diameter winding. Also, two grids of same diameter, but one fine in pitch and the other coarse, may be used. The variable mu characteristic can, also, be obtained by tapering the screen as pointed out heretofore. The operation of the tube shown in Fig. 4 should be readily understood from the previous explanation. With high bias upon each grid section I2 and I4 only one of the grid sections has control over plate current, the grid section having control serving to modify the impressed signal. Obviously, grid I2 must be the grid section which operates at high bias since at low gain the signal is impressed on this grid only. At high gain, that is to say with low grid bias, both grid sections I2 and I4 operate, and the signal is impressed on both.

The tapered signal control grid construction in Fig. 4 may be replaced by a construction such that the grid sections I2 and I4 are co-planarly arranged in either variable Inu or standard screen grid tube construction. In the latter case the screen grid would be tapered. In Fig. 5 there is shown still another practical construction for tube I0 which may be utilized for this purpose in place of the tube construction shown in Fig. 4. A schematic representation is employed for this modification in order to render the present disclosure simple. The electrodes are supported by three parallel spaced mica discs 39, 3l and 32. The plate 49 is disposed between mica discs 32 and 3|, and the lower peripheral portion of plate 4I) is provided with hooks 33 to anchor plate 40 to the intermediate mica disc 3l.

The plate 4I is disposed between mica discs 3| and 30. The screen grid electrode 34 is wound upon supporting rods 35 which extend through the two parallel mica discs 3| and 32. The signal control grid section is similarly wound upon the supporting rods 3S. The supporting rods of the lower section of the grid 42 and of the screen that is between mica discs 30 and 3|, go out through the front and rear of the tube construction, so that they do not show any cross-section and for this reason only the upper supporting rods 35 and 3B are shown. The two sections of the screen may be connected together, or may have separate leads.

Grids 43 and 42 are of uniform diameter; one having a ner mesh, or smaller winding pitch than the other. This gives the same effect as a tapered grid construction, and as a matter of fact is easier to manufacture. The plate is divided into two sections, and all the electrodes are separated and spaced by the three mica discs. The side rods 35 and 36 project through apertures in the mica discs 3l and 32, and similar side rods project through apertures in the mica discs 30 and 3l but the side rods of the upper and lower sections are displaced by and therefore, do not interfere with each other. It is to be understood that the grid sections in the case of tube ID can be connected together externally when only the divided plate construction is desired, and conversely the divided plates can be externally connected where only the divided grid construction is desired. It will thus be appreciated that these tube constructions are readily interchangeable in function.

Returning again to the circuit diagram shown in Fig. l, and considering now more specically amplier tubes I6 and I5', it will be observed that they amplify the signals passing through 75 the channels of different selectivity electrically associated therewith. It is pointed out that this type of network is shown in the intermediate frequency ampliiier of Fig. 1 in order to demonstrate that the present invention is capable of Wide variation. The gain of each of these transmission paths is regulated by the AGC connections, and the outputs of each of tubes I6 and I6 is impressed upon the grids I8 and I9 of tube II. The tube is a. divided grid-single plate tube upon which is impressed the combined output of the two signal transmission channels.

The construction of tube I'I may assume various forms, just as in the case of tubes 9 and IIJ. As pointed `out heretofore, there may be utilized for tube II a tube constructedin the manner shown in connection with tube Ill, the divided plates being connected together externally to furnish the circuit associated with tube I'I. However, in Fig. 6 there is schematically shown an electrode construction which may be used for tube I'I. It will be seen that this tube construction is quite similar to that shown in Fig. 4, with the exception that the plate 50 is not divided, and the screen grid 5I is not provided with an electrostatic shielding ring as in the case of Fig. 4. The signal control grid sections I8 and I9 are arranged in tapered manner. In this way a variable mu characteristic is imparted to tube Il. It is not believed necessary to explain the functioning of tube Il, since this should be clear from the explanations given in connection with tubes 9 and I0.

In-Fig. '7 there is shown still another modified type of tube construction which may be utilized to provide any of the tube circuit arrangements shown in connection with tubes 9, Ill and I1. The electrodes of this modification are schematically represented, and they representk two matched variable mu tube elements, so designed, that if placed end to end they would work as a full-sized variable mu tube. It will be noted that the signal control grid G1 of one of the tubes has a narrower tapered diameter than the other grid G2. The various leads from the electrodes of the tubes'` have been lettered to denote the plates P1 and P2; the screen grid leads are denoted by the symbol S, the cathode lead is denoted by the symbol C. The heater leads for the internally heated cathodes of the two, tubel sections are denoted by the reference letter H.

By virtue of the electrode construction'the tube shown in Fig. 7 can be made to operate in the same manner as the tube in Fig. 4, the portion containing G1, corresponding to the upper part of Fig. 4 (I I' and I2), and the portion containing G2 corresponding to the lower part of Fig. 4, (I3 and I4). By providing separate leads from the two grids and the two plates, it is possible to utilize the tube construction shown in Fig. 7 for any of the purposes shown in connection with tubes 9, I0 and I'I of Fig. 1. It is believed that the manner of connecting a tube of the type of construction shown in Fig. 'I` will be clear to anyone skilled in the art from the aforegoing discussion of the various tube constructions and the utilization in the circuit of Fig. l. Any other variable mu construction can be used in place of the conical grids. For example, cylindrical grids of different pitches, or of variable pitch, or conical screens may be used.

It is within the scope of the present invention to utilize more than two plates within a single electron discharge tube of the variable mu type,

or to utilize more than two signal input grids,

" high gain.

both for the purposes to which tubes 9, I0 and I1 have been applied. Thus, there is shown in Fig. 8 a portion of a signal amplier system, and it is to be understood that this may be a section of the intermediate frequency ampliiier system of a superheterodyne receiver. The first tube V1 is of the variable mu type which includes a'plurality of plate electrodes. Merely by Way of illustration the plate of the tube has been shown as divide-d into four sections. A common signal input grid is utilized, and it will be understood that the variable mu characteristic can be obtaine-d in any fashion disclosed heretofore. For example, the signal input grid may be given a conical configuration, or the spacing between windings may progressively decrease along the axis of the grid.

The following tube V2 is shown as having its signal input grid divided into four sections, each section corresponding to its respective plate section of tube V1. A common output plate is used in tube V2, While the tuned coupling networks. M11, M12, and M13, and M14 couple each plate section of tube V1 to its respective grid section of tube V2. The grid sections of tube V2 may be constructed along any of the lines shown in the modifications disclosed hereinbefore. By way of example, it is. pointed out that the four sections rnay be provided from a single grid of variable pitch. Of course, a tapering grid may be divided into four sections. As explained before, the coupling magnitude of each of the coupling networks between tubes V1 and V2 is definitely correlated to the geometry of thetubes V1 and V2. V1 and V2 may be a tube with grid and plates both divided; all grids being connected together to make V1, plates being connected to make V2. 1

The signal input grid circuit of tube V1, and each of the signal grid circuits of tube V2, are connected to a source of variable negative grid bias A, as shown in connection with Fig. 1. This variable bias source may be automatically operated in accordance with signal amplitude variation, or may even be manually adjustable. In this way, the transmission characteristic of the coupling network between tubes V1 and V2 may be gradually varied as the negative grid biases are varied. It should be understood that it is within the scope of the present invention'to secure vthe characteristics shown in Fig. 2, or conversely, to provide an amplifier which has high selectivity with low gain, or low selectivity with The present invention is not restricted to the coupling devices shown in Fig. 1 or in Fig. 8. That is to say, the couplings between the amplier tubes may be provided by devices other than transformers. For example, there is shown in Fig. 9 `a coupling network between the signal input amplier 62 and the variable mu tube 65 of the divided grid type, which coupling network comprises combined transformer and condenser coupling. The transformer M20 has its tuned circuits each resonated to the operating signal frequency, and the co-eflicient of coupling between the tuned windings of the transformer is less than critical coupling. A sharp selec tivity characteristic is thereby imparted to the coupling network, with respect to the signal voltage developed across the secondary circuit and impressed on grid 66 of tube 65. The selectivity characteristic with respect to the voltage across the primary tuned circuit is a flat top curve, or a double humped curve, by virtue of the reaction-s of the secondary tuned circuit on the primary circuit.

The signal input to grid Si therefore has a broad band frequency characteristic. Grids 6| and Gil are connected to the gain control bias voltage source, and at high bias only the signal on grid @il is effective, and there is transmitted to the output of tube 65 a broad band of frequencies. With low bias, that is with weak signal reception, the grid Gil has a predominant effect, and the output of tube 65 contains a narrow band of frequencies., f course, at intermediate bias settings intermediate frequency selectivity characteristics are obtained. The modification shown in Fig. 9 is independent of the nature of the variable Inu tube 65, and it is to be understood that any of the variable mu. tube constructions disclosed hereinbefore can be emplayed in that position.

Fig, 1) shcws a modified form of the invention, specifically applied to tube E5 of Fig. 9, and Wherein the bias for the two grids 6| and 60 is obtained by varying the voltage of the cathode of tube 65 with respect to ground. This is accomplished by connecting the cathode lead to an adjustable tap il which is slidable over a grounded resistor 08. Grids Si and E0 are grounded, and it will therefore be seen that variation of the position of slidable tap 'i on resistor 68 will vary the negative bias of each of grids 5| and 60. The tap 51 is manually adjustable; there is thus provided an arrangement for manually Varying the fidelity characteristic. It is obvious that any of the coupling networks to tube 55, shown in a previous portion of this specification, may be used in place of that shown in Fig. l0. Furthermore, any of the variable mu tube constructions hereinbefore disclosed may be used in place of tube S5. Ir. general, the results of the present invention may be secured either automatically or manually by varying the bias of the signal control grids. In the case of automatic control a common source automatic gain control voltage may be used, or independent control voltage sour-ces may be employed. Again, the nature of the coupling network between amplifier tubes may be varied as shown in connection with Fig. 9.

Another modification of a coupling network between amplifier tubes is disclosed in Fig. 11. In this case the numerals i0 and 1| denote, in general, a pair of amplifier tube constructions. It is to be understood that these two representations may designate separate tubes of the 58 or pentode type or they may be of the type shown in Fig. '7 where the Sections are independently biased. The significance of Fig. 1l resides in the particular construction of the coupling network between the amplifiers l0, 'il and the output amplifier 90.

The amplifier "it includes in its output the circuit Bii which is tuned to the operating signal frequency; the amplifier il includes in its output the tuned circuit Bl which is resonated to the operating signal frequency. The signal tuned circuit S2 is connected to the input of amplifier and is coupled to the circuit 8| by the coupling Mio which is more than critical. The coupling between circuits 80 and 8| is denoted by the syinbcl lvso and is less than critical. Thus, the selectivity characteristic between circuits 80 and 8i is sharp, and the characteristic between circuits 8l and 82 is broad. The coupling between circuits 8:3 and 82 is substantially reduced to zero by proper location of the coils of these circuits, and/or shielding, or proper lbucking coupling windings. Any of these devices is known to those skilledin the art, and may be utilized to keep the coupling'magnitude between circuits 80 and 82 substantially at Zero.

The input circuits of amplifier sections 'l0 and '1| are connected to the source of variable negative voltage, and as explained heretofore, this may be a manually or automatically regulated source of voltage. Specifically the source has been denoted by the symbols AVC tol denote that it is automatic in response to received signal amplitude variations. In order to secure a sharp selectivity characteristic, as when receiving weak signals, the amplifier section 10 is operative and transmits the signal energy to circuit 80, while the amplifier section 1| is biased olf. Therefore, the loose coupling Mao imparts the sharply selective vcharacteristic to the network, even though the coupling Mio is more than critical.

On the other hand for broad tuning the amplifier section'l is biased off, and ampliner '1| is permitted to amplify the signal. In that case the close coupling Mio is operative, and imparts the broad band characteristic to the coupling network. For optimum results the amplifier section 'il should have a low Rp, or sufficient damping should be used in circuits 8| and 82, and circuit 80 merely helps slightly broaden the characteristic curve and hold down the amplification at the center of the curve. The operation of the amplifiers l0, '1| from a common source of AVC voltage should be clear from the preceding description and Fig. 1, where such common souice is desired.

In Fig. 1 the couplings M6 and M7 have been shown connected to different amplifier tubes or load circuits so as to divide the signal between two outgoing channels. The operation of the gain control bias then causes the signal to flow in both channels when the bias is low; through solely one channel when the bias is high; through one channel with practically normal intensity, and through the other channel with reduced intensity, when the bias is between its extreme values. Such an arrangement can be employed ina double loud speaker system in which the two speakers handle different frequency bands. In such a case the input transformers are audio frequency Vtransformers instead of radio frequency transformers. Such an audio frequency network may also be used in a system having combined gain and tone control with the double output 'combined and fed to a single loud speaker. In this case the low gain circuit may have high peaks at low and high frequencies, and the high gain circuit a nat response curve. This combination would then give the effect commonly sought for in tone control circuits which aim to vary the fidelity in accordance with loudness to match the characteristics of the human ear. Of course push-pull circuits could be employed in such an audio amplifier system, if desired.

In Fig. 12 there is shown such an audio frequency transmission network wherein the transformer |00 has its primary winding connected toA any desired source of audio frequency input energy. The tubes |0| and |02 are each triodes of the variable mu type, and it will be observed that they are both of the divided plate type. The specific construction of each of these tubes may follow the form of Fig. 3 if desired. The signal input grids of tubes |0| and |02 are connected to opposite sides of the secondary winding of input transformer |00, and there is provided a variable source of gain control voltage for the tubes. This gain control voltage source comprises the current 75` source |04 which has connected thereacross a resistor 105. The positive side of resistor 105 is connected to the common cathode lead of the two tubes, while the center tap of the secondary winding of transformer 100 is connected'to an adjustable tap 103 which is slidable over resistor 05.

A pair of loud speakers are provided for the two tubes, and loud speaker LSI is coupled to plates Pi and P"1 of tubes li and |02 respectfully, through transformer T1. Loud speaker LS2 is coupled to plates P2 and P2 through the coupling transformer T2. In each of the two tubes shown in Fig. 12 the plates P1 are plates which cut off rst when the bias voltage on the input grids is increased; whereas the plates P2 are those which operate after the other plates have been cut off. Suitable power ampliiiers may be interposed between T1 and LSE, and between T2 and LS2, tubes lili and HB2 operating then as voltage amplifiers of small power output.

The audio frequency transmission characteristic of the network including transformer T1 and its associated loud speaker is denoted by the convex curve Ti in Fig. 13. It will be observed that this characteristic has a convex shape between 100 and 10,000 cycles. The audio transmission characteristic of the network comprising transformer T2 and its associated loudspeaker is represented by the curve T2 of Fig. 13. This curve has peaks at the low and high ends of the audio transmission characteristic.

'Ihe adjustable tap |03 is regulated to adjust the amplification of tubes l and E02 when the two audio systems operate as an acousticallycompensated system, the eXtreme high and low audio frequencies becoming relatively stronger in comparison with the middle frequency as the amplification and output are reduced. 'Ihe arrangement in Fig. 12 is not restricted to the particular circuits or variable mu type tube shown, but any of the other tube constructions or circuits disclosed hereinbefore may be utilized for this purpose.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. A method of controlling the transmission of a pair of parallel signal transmission channels of diierent frequency response characteristics which include at least two electrode sections of dierent gain control characteristics, which comprises the step of varying the space current flow in each section with gain control biases of equal magnitude whereby the signal transmission through said channels is varied at different rates.

2. A method of controlling the signal transmission through a pair of transmission channels of diierent frequency response characteristics, each of which channels includes an amplifier and the amplifiers of the channels being of different gain control characteristics, which consists in deriving a gain contro-l voltage from incoming signals, applying said voltage to said amplifiers with the same magnitude to vary the transmission through said channels at different rates thereby causing one of the response characteristics to predominate over the other.

3. A method of controlling the signal transmission through a pair of transmission channels of substantially opposite frequency response characteristics, each of which channels includes an amplifier and the amplifiers of the channels being of different gain control characteristics, which consists in deriving a gain control voltage from incoming signals, applying said Voltage to said ampliers with the same magnitude to vary the transmission through said channels at different rates thereby causing one oi the response characteristics to predominate over the other.

4. A method of controlling the signal transmission through a pair of transmission channels of substantially inverse frequency response characteristics, each of which channels includes an amplifier and the amplifiers of the channels being of different gain control characteristics, which consists in deriving a gain control voltage from incoming signals, applying said voltage to said amplifiers with the same magnitude to vary the transmission through said channels at different rates, and combining the signal loutputs of the channels in a common utilization network.

5. In combination with a source of signals and a demodulator, an ampliiier network having its input coupled to the source and its output coupled to the demodulator, said network comprising at least two parallel signal transmission circuits of different frequency response characteristics, said circuits having gain control characteristics which are different, and means for varying the gain oi each circuit whereby the signal transmission through said channels varies at dilerent rates.

6. In combination with a source of signals and a demodulator, an amplier network having its input coupled to the source and its output coupled to the demodulator, said network comprising at least .two parallel signal transmission circuits having different selectivity characteristics, said circuits having gain control characteristics which are different, and means for varying the gain of each circuit whereby the signal transmission through said channels Varies at different rates.

'7. In combination with a source of signals and a demodulator, an ampliiier network having its input coupled to the source and its output coupled to the demodulator, said network comprising at least two parallel signal transmission circuits having substantially inverse signal selectivity characteristics, said circuits having gain control characteristics which are different, and means responsive to variations in signal amplitude for varying the gain of each circuit whereby the signal transmission through said channels varies at different rates.

8. In combination with a source of signals and a demodulator, an amplifier network having its input coupled to the source and its output coupled to the demodulator, said network comprising at least two parallel signal transmission circuits, said circuits having gain control characteristics which are different, and means for varying the gain of each circuit with biases of equal value whereby the signal transmission through said channels varies at different rates, each of the parallel circuits including an electron dscharge device having a variable mu characteristic.

9. In combination with a source of signals and a demodulator, an amplifier network having its input coupled to the source and its output coupled to the demodulator, said network comprising at least two parallel signal transmission circuits, said circuits having gain control characteristics which are diiierent, and means for varying the gain of each circuit whereby the signal transmission through said channels varies at different rates, the parallel circuits being of inverse selectivity characteristics, and each circuit including an electron discharge device of the variable mu type.

l0. In combination with a source of signals and a demodulator, an amplifier network having its input coupled to the source and its output coupled to the demodulator, said network comprising at least two parallel signal transmission circuits, said circuits having gain control characteristics which are different, and means responsive to variations in signal amplitude for varying the gain of each circuit whereby the signal transmission through said channels varies at dilferent rates, the parallel circuits being of inverse selectivity characteristics, and each circuit including an electron discharge device of the variable mu type.

11. In combination with a source of signals to be amplied and a utilization network, a pair of tubes having a common signal input circuit coupled to the signal source, each tube having a pair of electrode sections of different control characteristics, said network including at least two circuits of different frequency response characteristics, the outputs of like pairs of said sections of said tubes being connected to a predetermined one of the utilization circuits.

12. In combination a pair of parallel signal amplifying channels, a source of signals feeding said channels and a common utilization network coupled to the output of said channels, said channels having at least one common electron discharge tube which is provided with at least two electrode sections which have diierent control characteristics, and means responsive to signal amplitude variations for impressing substantially equal gain control grid biases on the two sections whereby a change in bias has a greater effect on the signal transmission through one of the parallel channels than through the other.

13. In combination with a source of audio frequency signals, an electron discharge tube provided with an input circuit coupled to said source, said tube having a plate electrode divided into at least two parts, at least two audio signal channels having different and suitably related frequency response characteristics, one of the plate parts being connected to one of said channels and the other part being connected to the remaining channel, the geometry of the tube being such that the gain from the signal input grid in the two plate circuits varies differently as the grid bias of the tube is changed, and means for varying the signal input grid bias of said tube.

14. A signal transmission network comprising at least two parallel signal circuits and a tube provided with a divided plate, each plate section being connected to a different one of said parallel signal circuits, the electron streams to the plate sections having different amplification factors, and means for varying the electron ow to the plate sections of said tube whereby the transmission through the parallel circuits varies at different rates.

l5. In combination with a source of signals and a common utilization network, at least two parallel signal transmission channels, an electron discharge tubey common to both channels, said tube being provided with a cathode, plate and a divided input grid said plate being connected to the utilization network, and the signal channels being connected to impress signal voltage on the divided grid, said divided grid being so constructed that one of the grid sections has greater control over the electron current flowing through it than the other, and means for varying the grid bias of said tube in response to signal amplitude variations whereby greater changes in grid bias produce a larger change in amplification with respect to one section of the grid than with respect to the other.

16. In combination with at least two transmission paths for alternating current energy of different frequency limits, said paths having different frequency response characteristics, an electron discharge repeater device in each path, the repeater devices having different amplication factors, and means for adjusting the gain of said devices with control biases of equal value.

RENE A. BRADEN. 

